1. Field of the Invention
This invention relates to the production of a granular carrier material for pesticides from attapulgite, and to a carrier material produced according to the method.
In this specification "attapulgite" means both pure or high grade attapulgite and impure or low grade attapulgite. "Impure attapulgite" includes minerals which comprise a major proportion of attapulgite, together with substances, e.g. magnesium and calcium carbonates, which can be regarded as impurities, because they reduce the absorbency of the impure attapulgite, when compared with pure attapulgite. This impure or low grade attaplugite is regarded commercially and industrially as unsuitable for use as a carrier material, as explained hereunder, for solid state pesticides, when compared with high grade attapulgite.
One of the useful properties of attapulgite is that when dry, it absorbes water to a lesser or greater extent, depending on its purity. This renders it useful when it is desired to absorb liquids. For example, attapulgite may be used to form cat litter, or it may be used as a carrier material for pesticides in agriculture, the latter being an important commercial application in which use is made of the absorbency (sorptivity) of attapulgite with respect to liquids.
When attapulgite is dried by heating up temperatures of say, between about 150.degree. and 200.degree. C, it undergoes no structural change. Thus, when water is added to the dried attapulgite, it reverts to its original state.
On the other hand, when attapulgite is heated or fired to temperatures between about 250.degree. and about 800.degree. C, it loses further water but at the same time undergoes an irreversible chemical change. The water which is driven off between about 250.degree. and about 800.degree. C comprises chemically bound water or the constituents thereof, which are present in the original attapulgite in the form of, principally, lattice water, water of crystallization and hydroxyl groups. Upon heating to between about 250.degree. and about 800.degree. C, the attapulgite undergoes an irreversible change, although its structure, apart from the loss of the chemically bound water, remains substantially unchanged. Whereas attapulgite which has not been fired forms a more or less plastic mixture with water and forms a suspension in water, the fired product does not do so and separates rapidly and easily from water in which it is dispersed. Furthermore, the absorbency with respect to liquids of the fired product, particularly when fired to temperatures in the range between about 500.degree. and 800.degree. C, is superior to that of the original attapulgite.
Finally, when attapulgite is heated to temperatures above about about 800.degree. C, it undergoes a irreversible chemical and structural change, which is complete at temperatures above 1000.degree. C, in which its physical and chemical properties are altered, the product having an absorbency with respect to liquids which is less than that of the original attapulgite.
In agriculture, solid state pesticides are extensively used for pest control. Solid state pesticides comprise liquid toxicants, e.g. insecticides, herbicides, miticides, fungicides and rodenticides, and a solid granular diluent or carrier material, the toxicant being absorbed into the carrier material.
Generally, the toxicant makes up between 1 and 40% by weight of the pesticide, the carrier material making up the balance. A suitable carrier material is preferably chemically inert with respect to most commonly used toxicants and has the following desirable properties:
Granularity -- The carrier material should be capable of forming small granular particles, so that the carrier material in bulk is a free flowing granular solid. PA1 Stability in use -- The carrier material should be of sufficient density for particles thereof to remain in position on soil or foliage to which they have been applied in windy conditions; and should be water-resistant in that they resist deterioration and do not physically decompose or collapse into a runny sludge in rainy conditions. The carrier material should also be substantially unaffected by changes in temperature and humidity encountered in transportation, storage and use. PA1 Absorbency (Sorptivity) -- The carrier material should be capable of absorbing the prescribed amounts of toxicants for each intended application; and it should at the same time remain a free flowing and relatively dry feeling granular solid when the toxicant is absorbed therein. Absorption should take place rapidly and easily on contact of the carrier material with most toxicants, and the carrier material should be easily wettable by toxicants. PA1 Compatibility -- The carrier material should be both physically and chemically compatible with most toxicants, i.e. the carrier material should not react chemically with the toxicants or deteriorate physically when in contact with them, and it should not cause deterioration of the toxicants. PA1 Retentivity -- The carrier material should release the pesticide to the atmosphere at a controlled, uniform and preferably slow rate, thereby to achieve vaporization of the toxicant at a controlled release rate over a predetermined period of time. PA1 Resistance to Attrition -- The carrier material should resist attrition so that dust generation is kept within allowable limits during normal handling in transport, storage, application and use. Dust can adversely affect the bulk handling of the carrier material and can be lost in application and use in windy conditions. PA1 a. wet mixing perlite and attapulgite together in an aqueous medium to form a cake which comprises an intimate mixture of between 97 and 80% by weight of attapulgite in finely divided form and between 3 and 20% by weight of heat expanded perlite in finely divided form, the attapulgite and perlite being of a particle size capable of passing through a 150 mesh ASTM screen; PA1 b. heating the cake to a temperature of between 450.degree. and 780.degree. C; and PA1 c. breaking up the cake to form a granular solid.
In practice the carrier materials is frequently shipped in bulk to compounders of pesticides, and a compounder may wish to make several different types of pesticides, depending on the toxicant used, from the same carrier material. A good carrier material should thus be capable of use with most toxicants and the applicant has found that, to be a commercially practical product, the carrier material should be capable of absorbing a test liquid (defined hereunder) so that the test liquid makes up about 30 to 40% of the pesticide product.
In the past, high grade (with respect to absorbency) attapulgite has been extensively used as a carrier material. Attapulgite is a clay which is found in deposits in relatively few localities, principally in Georgia, U.S.A. of sufficient quality to be used as a carrier material. There are other large deposits, in the U.S.A. and elsewhere, of low grade attapulgite, which are not suitable for use as a carrier material, having an absorbency which is too low to meet the more exacting applications. The invention thus finds particular application in increasing the absorbency, after firing, of low grade attapulgite, to levels acceptable for use as a carrier material.
2. Description of the Prior Art
In the past, it has been known to increase the absorbency of attapulgite for use as a granular carrier material. The prior art discloses the heating or calcining of attapulgite to temperatures between about 250.degree. and about 800.degree. C, the attapulgite losing water and undergoing an irreversible chemical change. A product is obtained which is substantially superior to the unfired attapulgite, as an absorbent of pesticides, for use as a granular carrier material. The absorbency of the fired product with respect to liquids is increased, as is the resistance to attrition thereof; and it forms a suspension less readily in water than the original attapulgite, and separates more easily from water in which it is dispersed.